This device relates to a manhole cover for tanks and containers which hold flammable liquids and are mounted upon vehicles for the purpose of transporting such liquid cargo.
Tanks for storage and transport of volatile and flammable liquids, such as gasoline, contain manhole covers for filling and for interior access. Such tanks and containers are provided with conventional vents through which fumes and vapors which normally accumulate within the tank can escape to atmosphere and thereby prevent build-up of excessive pressure in the tank. Failure to provide such venting can result in dangerous pressure build-up and a possible explosive condition. However, in the event of fire or sudden temperature elevation, such conventional vents do not provide sufficient relief for rapid temperature increases, and "emergency venting" is required.
In the prior art, such emergency venting has been accomplished in various ways. In U.S. Pat. No. 3,292,814 to Howard Krone, the cover of the manhole is mounted on a hinged yoke. The yoke and latch are fusibly mounted to a coverplate. The fusing alloy has a melting point such that the pressure of excessive heat will melt the alloy, thereby releasing the yoke and cover to permit emergency venting.
In U.S. Pat. No. 3,179,285 to John DeFrees, the cover itself is fusibly mounted on the yoke.
A disadvantage of fusible emergency venting is the possible absence of sufficient temperature in the immediate proximity to the fusible alloy to cause melting. Another disadvantage is that, once the fusibly alloy has melted, the venting device cannot return to a seated position, and flammable product continues to flow from the manhole.
Another means of emergency venting, as shown in U.S. Pat. No. 3,339,791 to DeFrees, depends upon a spring-mounted cover. When internal tank pressure reaches a predetermined level sufficient to overcome the pre-loaded spring force, the cover will unseat and effect emergency venting. When excess pressure subsides, the spring will force the cover back into a seated position.
The major disadvantage of pressure actuated venting arises in situations of tank overturn. At impact, a pressure surge generated by the liquid cargo against the cover far exceeds the pressure necessary to vent the cover. Such excess pressure forces volatile and combustible product through the restricted opening of the partially unseated cover and creates an outwardly expanding cloud of volatile mist. Any flame or spark in the vicinity of the cloud could cause ignition and a resulting fire.
Another disadvantage of prior manhole covers is the sealing arrangement between the coverplate and collar which extends upward from the tank. Frequently, a flat seal is interposed between them and depends on the clamping force of a circumferential ring to effect sealing. Such a sealing method has been demonstrated to be unreliable in overturn accidents, permitting leakage of flammable product past the seal and into the environment.